Abstract: There is no clear formation mechanism of hydrogen porosity in galvanized steel and aluminum alloy dissimilar joint. The spillover mechanism of hydrogen porosity of AlMg5 wire cladding metal on surface of galvanized steel plate was investigated. The galvanized steel plates with different thickness were employed to control the cooling rate of cladding metal.The effect of Al₃Fe IMC and holding time above the liquid phase temperature on the porosity spillover behavior was studied.The results show that the flaky Al₃Fe IMC hinders the spillover process of porosity and contributes to the formation of irregular porosity. The excessive cooling rate shortens the holding time above the liquid phase temperature of the deposited metal and also limits the spillover of porosity.